Disposable plastic straw-insertable lids are well known, and are found in environments where beverages are vended, dispensed or sold for consumption, such as at fast food restaurants. The straw-insert opening facilitates the insertion of a straw into the cup without eliminating the protection provided by the lid against spillage of the cup's contents.
Typically, a straw insert opening comprises two perpendicularly crossing "straw insertion slits" in the lid body. The straw insertion slits may be lines of weakness or may be complete perforations in the lid. The slits create a pattern in the lid of four adjacent wedge-shaped sections with their apexes at a common center point. A drinking straw is pressed against the center point, causing the wedge-shaped sections to be displaced inwardly, thus allowing the straw to pass through the lid and into the cup.
Conventional straw-insertable lids are subject to tipping, which occurs when the application of force during the insertion of a straw actually tears the lid rather than merely displacing the wedge-shaped sections defined by the lines of weakness. Typically, tipping occurs at the ends of the straw insertion slits and extends across the lid in the direction of the straw-insertion slits.
Major consumers of these lids, such as fast-food restaurant chains, demand that the tendency of the lids to rip be minimized. This requirement in the market exists for several reasons. First, tipping of the lid on insertion of the straw is undesirable in that it provides an enlarged opening through which liquid will flow if the cup is tipped, jostled, or overturned. Second, this type of tipping may produce a jagged edge which is sharp and therefor poses a safety hazard. Finally, tipping changes the geometry of the straw insertion slits so that the straw may not be located with its central longitudinal axis at the intersection of the straw insertion slits. When the straw is correctly located, the forces applied to the straw by the wedge-shaped portions of the lid are uniform forces and thus do not tend to crush the straw, but are resisted by the straw's uniform cross section. Mislocation of the straw as a result of tipping results in uneven forces tending to crush the straw and prevent the passage of liquid through the straw. This tendency of the tipped lid to crush the straw is particularly undesirable when a thicker beverage such as a milkshake is being consumed through the straw, since the crushing of the straw may effectively prevent suction of the milkshake through the straw.
The amount of tipping experienced varies with the type of plastic used in the cup lid; more flexible plastics (i.e. having a greater rubber content) such as K-resin crystal tend to rip less than more brittle or rigid plastics, such as high-impact polystyrene. Plastics such as high-impact polystyrene can be produced at lower cost, so it would be desirable to develop a lid design that would not rip even when constructed from high impact polystyrene.
An example of a known lid constructed to reduce ripping of this type is shown generally in FIG. 1. As shown in FIG. 1, lid 101 has a rim 102 which engages a cup rim (not shown). Straw insertion slit 103 comprises lines of weakness or through-slits 104 and 105, which are formed in relatively perpendicular fashion in the lid by stamping or molding. Transverse tip-stop end cuts 106 formed continuously with and perpendicular to the ends of through-slits 104 and 105 tend to reduce the extension of rips or tears past the ends of the through-slits. Stamping of numerous lines of weakness in the lid, as performed in designs of this type, results in a weakening of the planar structure of the lid in the region of the straw-hole. This weakness may result in cracking of the lids during packing and bulk shipment, and also in much less effective sealing of liquids inside the cup.
Another example of a prior art drinking lid device is disclosed in U.S. Pat. No. 4,948,009 to Sawatani, which shows a lid with a protruding outer ring which surrounds the otherwise conventional through-slits. The ring is said to stop fractures starting at an end of a through-slit. Designs of this type have the disadvantage of requiring more material than planar lids. Because these lids are produced in tremendous volumes, the need for even a small amount of additional material in each lid becomes expensive and thus undesirable.
Yet another type of straw-insertable drinking lid is shown in U.S. Pat. Nos. 4,245,752 and 4,350,260 to Prueher and U.S. Pat. No. 4,438,865 to Scattaregia. These references show holes provided at the ends of the intersecting through-slits which could perform a tear-reducing function. Of course, designs of this general type are less effective in keeping liquids inside the cup since liquids can pass through the holes.
U.S. Pat. No. 4,502,608 to Mills discloses a disposable lid for a drinking cup in which a cut is made through the thickness of the lid. The cut functions to terminate the tearing-out of a wedge-shaped piece as it is removed from the cup lid to permit drinking from the cup. U.S. Pat. No. 4,999,230 to Pipkins discloses a sheet with removable sections defined by a series of arcuate cuts which are separated by connection points. The connection points are broken to remove the sections from the sheet.
Further, with each of the above-noted lid configurations, should a filled cup having the lid thereon tip over, the pressure of the fluid within he cup will press against the inner surface of the wedge-shaped sections causing them to spread outwardly and away from one another. In doing so, the fluid can readily pass through the slits thus spilling from the cup. This is likewise true for lids having intersecting lines of weakness in that the fluid pressure may be great enough to overcome the strength of the membrane.
Additionally, when forming the straw slits in the above-noted lid configurations, the slits are formed by a cutting die at a station downstream from the lid formation station which requires that the lid be properly registered with the cutting die at that station. If the lid is not correctly aligned with the cutting die, the lid will be miscut resulting in a defective lid which must be scrapped, thus increasing the costs associated with the production of the lids.
As noted, known designs for disposable cup lids have not been entirely satisfactory. What is needed is a lid that could be manufactured from lower-cost, more brittle plastic, which would effectively hold in liquids prior to insertion of a straw, and which would not fracture in an undesirable manner during shipment, placement on the cup, or upon insertion of a straw. At the same time, an ideal design could be produced without requiring additional material or additional production operations, either of which would make the improved lid more expensive than known lids.